Week 1 - Peripheral Nervous System Flashcards
How are the different gradations of intensity transmitted?
- Frequency coding (temporal summation) - increasing the firing rate.
- Population coding (spatial summation) - increasing the number of parallel fibres that transmit information.
What is adaptation with respect to nerve stimulation?
What are examples of slow-adapting sensory receptors?
Fast adapting?
Adaptation: the intensity of a signal diminishes if the signal persists for a long time.
Slow adapting = Merkel cells and carotid bodies.
Fast adapting = Pacinian corpuscles.
What is lateral inhibition?
When a point is stimulated, neurons directly below point dampen the signal from adjacent neurons thus blocking the lateral spread of the signal.
What are the 4 elementary attributes of a stimulus?
- Modality/type
- Intensity
- Duration
- Location
What is a receptor/generator potential?
A depolarization of the receptive portion of the sensory axon generated by a sensory stimulus.
What nerves carry information on taste?
CN VII & CN IX
Somatic sensations can be divided into two groups - what are they?
- Exteroreceptors (sense change in external stumulus)
2. Interoceptors & Proprioceptors (provide info on body position)
Give an example of a interoreceptor/proprioceptor.
Ruffini’s corpuscle - sensitive to stretch of the skin; also senses position/mvmt)
Give examples of an exteroreceptor & briefly state what type of stimulus is detected by each of them.
- Pacinian corpuscle (deep pressure, vibration)
- Meissner corpuscle (light touch)
- Free nerve endings (pain, temperature, touch, pressure, stretch)
- Merkel disk (low frequency vibration)
What is neurulation? When does it occur?
Describe the process.
Formation, folding, and closure of the neural tube during the 3rd wk of development.
- the closure begins in the cervical region
- SHH stimulates the initial folding in of the neuroectoderm (ectodermal plate)
- as it folds, the borders of the neural plate become the neural crest cells
- the anterior neuropore closes at day 25 post fertilization
- the posterior neuropore closes 27 days post fertilization
- closure creates a lumen
In the development of the spinal cord, what determines the differences between doral and ventral? Briefly describe.
Concentration gradients and signalling cascades:
Dorsal edge = “roof plate”
- high concentration of BMP
- dorsal half of spine (Alar Plate) develop into sensory neurons
Ventral edge = “floor plate”
- high concentration of SHH
- ventral half of spine (Basal Plate) develops into motor neurons
Roof/Floor plates do not have any viable neurons.
What are the layers of the neural tube cell population that arise when the neuroepithelial cells proliferate?
(Start at lumenal side)
- Ventricular layer - very few nuclei
- Intermediate (Mantle) layer - thickest layer, lots of nuclei, develops into gray matter
- Marginal layer - develops into white matter of spinal cord
What happens when the neural tube fails to close?
Spina bifida:
- occulta (mild)
- cystica (mod) –> meningocele or meningomyelocele
- myeloschisis (severe) –> entire neural tube is open posteriorly
What embyological layer are keratinocytes derived from?
- what about melanocytes?
keratinocytes –> ectoderm
melanocytes –> neural crest cells
What is the etiology of neural tube defects?
Multifactorial:
- hyperthermia
- Valproic acid
- excessive Vit A intake
What is the best way to prevent neural tube defects?
Folic Acid intake, 0.4mg/day, before and during pregnancy
How are neural tube defects diagnosed?
US (8-10 wks)
AFP (maternal serum & amniotic fluid)
Describe the segmentation of the brain and state what each portion develops into.
- Prosencephalon (Forebrain) –> Telencephalon (cerebrum) & Diencephalon (thalamus, hypothalamus, pineal, post pituitary)
- Mesencephalon (Midbrain)
- Rhombencephalon (Hindbrain) –> Metencephalon (pons/cerebellum) & Myelencephalon (medulla)
What are the flexures of the brain?
Ventral flexures (at 5th wk):
- Cervical (brain-spinal cord jxn)
- Cephalic (midbrain)
Dorsal flexure (at 7th wk): - Pontine (hindbrain)
How does the pontine flexure affect the development of the brain?
It thins the roof plate and causes rearrangement of the Alar-Basal plates from a dorso-ventral pattern to a latero-medial pattern.
What is Wallerian degeneration?
Occurs after complete laceration of a neuron - the axonal section distal to the laceration will degenerate over a short time.
What is the body’s response to nerve cell laceration?
The proximal end of the lacerated axon makes growth cones and starts regenerating.
- sends out sprouts to search for neural tissue or laminin (which triggers regeneration)
- if the distal axon is found, the axon can follow that path to regenerate itself
- if the distal axon/neural tissue is not found, the proximal axon keeps sending out sprouts and may create scar tissue and develop a neuroma
When the laceration is surgically reattached, what happens if the ends are not trimmed prior to joining them together?
Can get scarring and neuroma formation.
What is carpal tunnel syndrome?
Compression of the median nerve at the base of the hand - get tingling/weakness/numbness in median nerve distribution
What is neurapraxia?
Temporary loss of sensory/motor nerve function due to nerve compression block in the peripheral nervous system.
- causes increased latency
What is the management of carpal tunnel syndrome?
Splinting and activity modification - goal is to provide largest volume in carpal tunnel to reduce compression of the nerve
What are microglia?
Macrophage cells in the brain
What is the name for stem cells in the brain?
- what can these develop into?
Polydendrocyte
- can develop into astrocytes, oligodendrocytes, or neurons
Where are bipolar neurons found?
retina
What does a nerve contain?
It is a bundle of fascicles containing many axons
What is an alpha motor neuron?
A motor nerve that directly innervates extrafusal muscle fibres (skeletomotor)
What is a gamma motor neuron?
A motor neuron that innervates intrafusal muscle fibres & helps keep the muscle spindle taut while the muscle is contracting.
Where are upper motor neurons?
They have their cell body in the motor cortex or brainstem.
- innervate lower motor neurons.
Where are lower motor neurons?
Cell bodies in spinal cord (anterior horn)
- directly innervate muscle fibers
What are the sensory receptors in muscle?
- Muscle spindles
- Golgi tendon organs
What types of muscle have a higher density of muscle spindles?
Muscles that are used for fine motor tasks (ex. hand muscles)
What is the role of the muscle spindle?
It informs brain about muscle length and velocity of contraction.
What are the three types of intrafusal muscle fibres?
- Bag1 (largest)
- Bag2
- Chain fiber* (contains a row of nuclei)
There are 4 chain fibers (and one of each bag fiber) in each spindle.
What is the sensory innervation to muscle spindles?
- Group Ia afferents (annulospiral ending; Bag1, bag2, & chain fibers) –> sensitive for detecting change
- Group II afferents (unmyelinated spray terminals; Bag2 & chain fibers) –> give info on length
What is the motor innervation to muscle spindles?
Gamma (fusimotor) neurons:
- static (bag2 & chain)
- dynamic (bag1)
What information is gained from Group II afferents?
Length
What information is gained from Group Ia afferents?
Velocity & Length
How does muscle stretch affect the muscle spindle?
Sensory neurons around spindle fire when it gets stretched
- stretch-operated cation channels lead to AP generation
What property does the action of dynamic gamma-motor neurons affect?
Velocity sensitivity
What property does the action of static gamma-motor neurons affect?
Length sensitivity
When are spindle sensory neurons firing?
They are tonically active - fire APs when muscle is at resting length
- results in muscle tone (from tonic excitation)
Describe the Hoffmann Reflex (the stretch reflex without stretch).
Peripheral nerve is stimulated.
- sends impulse down motor portion to stimulate muscle (M-wave)
- sends impulse up sensory portion, through spinal cord, back down motor to stimulate muscle (larger, H-reflex)
What is a myotatic unit?
A connection of nerve pathways controlling a single joint.
What is reciprocal inhibition?
Relaxation of the opposing muscle during contraction of the agonist.
What type of lesion can causes flaccid paralysis?
Lower (alpha) motor neuron lesion
What does an upper motor neuron lesion present with?
Spastic paralysis (ie. reflex is intact, but patient has weakness and difficulty controlling muscles). - may also show Babinski's sign
What types of neurons are found in a peripheral nerve?
- somatic motor
- somatic sensory
- autonomic (visceral motor/sensory)
Describe the classification schemes for the peripheral axon.
Based on conduction velocity (usually describes motor neurons):
- A (fastest, largest diameter, myelinated) –> B (myelinated) –> C (slowest, unmyelinated)
Based on diameter (exclusively for sensory neurons):
- I (largest, fastest) –> II –> III –> IV (smallest, slowest, unmyelinated)
What is the basal lamina in peripheral nerve axons?
- what is it required for?
It is ECM (mainly type IV collagen) that surrounds the individual axon and myelin.
- contains laminin & fibronectin which interact with growth cone adhesion molecules needed to guide axon sprouts in axonal regeneration.
There are two types of Schwann cells. What are they?
- Myelinating (covers segments of axon)
2. Ensheathing (covers entire length of axon)
Do type C fibers have Schwann cells?
Yes! but the schwann cells do not produce myelin
What is neurapraxia?
Grade I Injury - Segmental Demyelination (mildest form of nerve damage)
- causes conduction block (temporary loss of motor/sensory innervation)
What happens when there is complete demyelination of a segment of an axon?
- slower AP
- Na+ channels redistribute themselves away from nodes of Ranvier
What factors are needed for remyelination?
Trophic factors & cytokines released by damaged Schwann cells and affected axons –> causes proliferation of undifferentiated Schwann cells
What does the amplitude (of waves in an EMG recording) give you information about?
Number of neurons activated
What does the latency (of waves in an EMG recording) give you information about?
The conduction velocity (if the path length is known)
What is CMAP (Compound Muscle Action Potential)?
An EMG investigation whereby an M-wave is generated (during muscle contraction) by peripheral stimulation of alpha motor neurons
What is axontmesis?
Grade II Peripheral Axon Degeneration:
- Disruption of nerve cell axon & myelin sheath
- no damage to schwann cell, endo/peri/epineurium
- Wallerian degeneration
What is neuorntmesis?
Grade III Peripheral Axon Degeneration:
- both the nerve and nerve sheath are damaged
- only partial recovery is possible
What growth factor is released at the growth cone in Wallerian degeneration?
- what is it responsible for?
Neuroregulin
- initiates Schwann cell dedifferentiation & proliferation
What is allodynia?
- what is another name for this?
Allodynia (aka dysesthesia) = pain evoked with a normally non-painful stimulus
What is hyperalgesia?
Hyperalgesia = extra pain evoked with a normally painful stimulus
How are reflexes affected by Upper vs Lower MN neuropathy?
UMN –> hyperreflexia & increased tone
LNM –> Hyporeflexia, decreased tone, & atrophy
What is the difference in presentation of nerve root damage versus peripheral nerve damage?
Peripheral nerve damage has a more discrete margin around the affected area.
What are the features of axonal neuropathy?
- usually affects sensory neurons
- slowly progressive
- length dependent
- loss of ankle jerks
What are the features of demyelinating neuropathy?
- affects mainly motor but also sensory neurons
- rapid
- arms & legs
- areflexia
What is an example of axonal neuropathy?
Diabetic neuropathy
What is an example of demyelinating neuropathy?
Guillain-Barre syndrome
What diagnostic tests should be ordered to diagnose peripheral neuropathy?
- Blood work (CBC, glc, TSH, Renal & liver panels, Vit B12, VDRL, HIV, Lyme, Connective tissue screen, serum immunoelectrophoresis)
- EMG
- Lumbar puncture (if thinking GBS)
- Heavy metal analysis
- Nerve bx
What type of neuropathy is suggested by an EMG showing decreased amplitude?
- what about increased latency?
Decreased amplitude ==> Axonal
Increased latency ==> Demyelinating
